In hydraulic fracturing, a viscosified brine- or KCl-based fluid is routinely introduced at a rate and pressure sufficient to fracture the formation. At first, the fluid leaks off into the rock matrix, building up a filter cake on the rock face. The filter cake then prevents fluid injected thereafter from leaking off significantly. The full force of the applied hydraulic pressure eventually comes to bear upon the rock face, causing the rock to part at the weakest point. As the fracture grows, additional fracturing fluid containing solid proppant materials is introduced. Following this treatment, as much as possible of the introduced fluid is recovered from the formation, but the proppant remains to prevent the complete closure of the fracture. The propped fracture creates a highly conductive channel extending from the well bore into the formation, making the reservoir more productive.
The conductivity of a propped fracture is dependent on the particle size of the proppant material and the residuum of polymer and gel left behind in the fracture after cleanup. Obtaining a very high viscosity in the gel has been preferred in the past and, to a large extent, still is preferred. More viscous gels allow placement of larger particle size proppants and higher concentrations of proppants without a screenout (the proppant bridges across the mouth of the fracture prematurely, preventing the further introduction of proppant). The fracture width normally is directly proportional to the viscosity of the fracturing fluid. However, the use of high viscosity fluids to place relatively large-size proppant material in the fracture can be counter-productive whenever the high viscosity inhibits the degradation of the polymer and gel leaving the proppant pack conductivity low in spite of its large particle size. Natural products, guar gums and their derivatives, hydroxypropyl guar (HPG), and carboxymethylhydroxypropyl guar (CMHPG) have proven to have a satisfactory compromise of properties: low-cost, relatively high viscosity when suitably crosslinked, and relatively low residue left in the proppant pack.
Chemical degradation by breaking agents which are deliberately added is a necessary part of modern hydraulic fracturing technology. It is usually delayed as long as possible through the use of various strategies, including the time-release approach. Long before the deliberately-added breaker begins its work, natural mechanical, chemical, and thermal polymer degradation processes are at work, breaking down the gel.
High viscosity fracturing fluids undergo high shear stress during the introduction of such fluids into a formation. The viscosity of the fluid must be high enough to carry proppant but low enough that excessive friction losses and high well head pumping pressures are not encountered. Polymer degradation is a natural result of shear stresses imposed by pumping, the presence of abrasive materials, and high flow rates through small flow channels. This mechanical degradation accompanies thermal degradation and chemical degradation produced by acid-catalyzed hydrolysis of the acetal bonds which are the weakest links along the guar, HPG, or CMHPG polymer backbone.
Gels based on CMHPG and crosslinked at pH values in the range of 3.0 to 6.0 are preferred in the industry because such gels have a reputation for excellent proppant pack clean-up. Gels crosslinked at pH values in the range of 3.0 to 4.5 are especially preferred. Carbon dioxide, when dissolved in water or brines, tends to drive the pH into the latter pH range. Gels crosslinked at pH values in the range of 3.0 to 4.5 are yet further preferred because of their natural tendency to be compatible with CO.sub.2. Such gels are useful not only in normal hydraulic fracturing but also in operations involving so-called energized fluids, foam-like or emulsion-like dispersions of high pressure CO.sub.2 or CO.sub.2 /N.sub.2 mixtures into a normal aqueous phase or one viscosified with polymer or crosslinked polymer.